Don't call controller's headers method internally

Since 5745a3c0928ee5604ce80af19348efb42189f1d6, if a controller defines
a `headers` method it will be called by this line, and the return value
will be mutated. This was also preventing the "Vary" header from being
sent to the client.

Co-authored-by: Oleksandr Bezruchenko <alex.bezruchenko@intercom.io>
Co-authored-by: Iliana Hadzhiatanasova <iliana.hadzhiatanasova@intercom.io>

actionpack/lib/action_controller/metal/rendering.rb

@@ -78,8 +78,8 @@ def _set_rendered_content_type(format)
       end
 
       def _set_vary_header
-        if self.headers["Vary"].blank? && request.should_apply_vary_header?
-          self.headers["Vary"] = "Accept"
+        if response.headers["Vary"].blank? && request.should_apply_vary_header?
+          response.headers["Vary"] = "Accept"
         end
       end
 

actionpack/test/controller/integration_test.rb

@@ -850,6 +850,30 @@ def app
   end
 end
 
+class ControllerWithHeadersMethodIntegrationTest < ActionDispatch::IntegrationTest
+  class TestController < ActionController::Base
+    def index
+      render plain: "ok"
+    end
+
+    def headers
+      {}.freeze
+    end
+  end
+
+  test "doesn't call controller's headers method" do
+    with_routing do |routes|
+      routes.draw do
+        get "/ok" => "controller_with_headers_method_integration_test/test#index"
+      end
+
+      get "/ok"
+
+      assert_response 200
+    end
+  end
+end
+
 class UrlOptionsIntegrationTest < ActionDispatch::IntegrationTest
   class FooController < ActionController::Base
     def index